After a rebuild my RM250 its blowing coolant out of the overflow tube, I know this can be a bad head gasket but after disassembly the gasket (o rings) are dry and show no signs of leakage. Leakdown test proves ok...holds 5 psi for 10 mins. But the more i think about it, a comp test shows 215psi clyinder pressure, this is way more than a 5psi leakdown test so how does a leakdown test prove if the headgasket is leaking at all? Should I order some new gaskets and try again? Any other ideas?

bad rad cap?

I tried that, first thing I swapped out I wish it were that simple